An implant generally refers to an artificial tooth that is obtained by inserting a screw made of titanium into alveolar bone and engaging a separate prosthetic tooth (or a correction tooth) with the inserted implant.
Recent developments in implant technology and problems of conventional implants will be discussed below.
First, a gap is inevitably formed where a prosthetic tooth (or a correction tooth) is connected to provide an environment for bacterial proliferation which may produce oral malodor together with the residual food in the gap.
Second, according to a known implantation method illustrated in FIG. 2, a bone 101 is cut with a surgical tool (or a drill) 110 to form a hole (or an aperture) 102, and an implant fixture 20 is implanted into the bone through the hole 102. Drilling itself is an operation that destroys and weakens the bone 101. However, technically drilling and making a hole are required for the implantation. Since the drill is basically different in shape from the implant fixture 20 to be implanted, it is absolutely impossible to accurately align the implant fixture 20 with the hole formed by drilling. If the hole formed by drilling is large, a dead space (or an empty space) 105 may be created lowering the primary stability, thereby increasing the possibility of implant failure. Meanwhile, if the hole is small, the pressure of the implant fixture applied to the bone increases, resulting in bone destruction. If the hole or aperture formed by drilling is short, the implant fixture may not be completely inserted into the hole. Meanwhile, if the hole or aperture is long, the dead space 105 may be formed under the implant fixture even when the implant fixture is fully inserted into the hole, and as a result, the bone cannot support the vertical force.
In the case where there is a risk that an implant fixture may be erroneously placed into a hole drilled in a false direction, a new hole must be formed by drilling. In this case, the bone may be severely destroyed, causing implant failure. It is thus believed that conventional cylindrical implant fixtures do not have any vertical drilling function, or if any, cannot perform a substantial vertical drilling function in spite of advertisements of their makers. This is because such a cylindrical implant fixture has a flat or convex tip and cutting blades thereof are distant from the center of a core, making it impossible to reduce the central portion of the alveolar bone in contact with the core. A conventional implant fixture without drilling function at the tip is not advancing smoothly when the direction of insertion is to be changed or when it is rotated with compression to place it deeper or obtain higher anchoring. In the state where moving forward is not accommodated, arbitrary rotation of the implant fixture causes complete cutting of the bone in an area where the cutting blades reach and leads to loosening of the implant fixture. Even when slight advancing is permitted, an excessive pressure is applied to an upper screw of the implant fixture where no cutting blade is provided. In the case where the implant fixture is not advanced reducing the alveolar bone but is forcibly pushed into the bone due to the excessive pressure by the rotation of the upper screw, the deeply seated implant fixture brings about bone destruction because the implant fixture is not inserted by bone resection of the tip but is pushed into the bone plate. As a result, dead spaces are formed at screw valleys which are not fully filled with the cut alveolar bone chips. In conclusion, the use of the implant fixture including the tip with no drilling function increases bone destruction. This situation is continuously worsened by bone compression at middle and upper portions of the implant fixture. In an attempt to overcome such problems, additional or frequent drilling of bone is performed leading to problems of increased bone loss and complicated surgical process, inherent with placing the conventional implants.
For reference, FIG. 1a illustrates an implant fixture of Korean Patent Application No. 10-2009-7024276 entitled “Dental implant”. The prior art aims to improve a condensing effect by increasing the thickness of the implant fixture 20 from a distal end 24 to a proximal end 22 thereof, but cannot achieve drilling or cutting effect because two spiral grooves 48 extend only to a middle portion of the implant fixture in the longitudinal direction of the implant fixture. Despite the effort to improve a condensing effect, an excessive force is applied to bone in the course of insertion of implant. In particular, since the implant fixture includes a flat shelf-shaped tip, the vertical self-drilling function of the implant fixture is so little that vertical penetration is impossible. Thus, this technique fails to solve the problems of implant fixtures invented before.
The change of direction of the dental implant is not allowed because of no vertical penetration or advancing function. The implant fixture has a two-body structure as implant is connected to a supra structure of an artificial tooth. Due to this structure, a fine gap is inevitably formed between a connection screw and the implant fixture. As a result, bacteria propagate and food residue is collected in the gap to produce an unpleasant smell.
In addition to the problems of the conventional implant fixtures during placement, problems associated with immediate loading and immediate placement will be described to propose improvements of implant fixtures. Dental implant placement can be classified into immediate placement and delayed placement. According to immediate placement, an implant fixture is placed immediately after a tooth is pulled out. According to delayed placement, an implant fixture is inserted after a predetermined period of time (about two months) following tooth extraction. Dental implants can also be classified into immediate loading and delayed loading. According to immediate loading, a dental prosthesis is immediately engaged with an upper portion of an implant fixture for mastication and aesthetic functions after implantation. According to delayed loading, a dental prosthesis is engaged with an implant fixture 3 to 6 months after implantation.
The functions of immediate loading can be divided into aesthetic and actual mastication functions. Primarily anterior teeth are for the aesthetic function and posterior teeth are for the mastication function. Particularly, immediate loading is further required for an aesthetic feeling of a patient. However, most implant fixtures known in the art are based on delayed loading after delayed placement. The reason why immediate loading cannot be applied to conventional implant fixtures is not because the implant fixtures are intentionally designed for delayed loading, but because existing designs of the implant fixtures are not suitable or disadvantageous for immediate loading. For this reason, various problems occur. In the course of delayed implantation and delayed loading, it is difficult to utilize a mastication function after implantation and the aesthetic appearance of the patient' face may be damaged, restricting the social life of the patient. Immediate placement and immediate loading are considered to avoid this problem, but they are impossible to perform in view of the physiological properties of human bones and the shapes of existing implant fixtures.
To elucidate the reason, it is necessary to understand the physiological properties of human bones to some extent. Human bones can be generally classified into lamellar bones and woven bones. Lamellar bones refer to natural bones that perform normal physiological functions. Woven bones are formed, for example, when human bones are damaged or stress fractured after long-term use or when hormonal changes occur. In post-menopausal women, woven bones are formed when bones are weakened due to a reduction in estrogen level or are fractured, or when bone resorption is accelerated to reduce bone mass or make bones brittle.
When holes are drilled into lamellar bones for dental implantation, the bones are changed to woven bones. Woven bones are weaker than lamellar bones, suffer from an abnormal resorption profile, or become very weak during regeneration. When additional stimulation is applied during immediate loading, woven bones may be further destroyed rather than being regenerated or may be converted into flesh (fibrous tissue; soft tissue) instead of bones (hard tissue) through fibrous metaplasia. This is because stem cells or mesenchymal cells are converted into bone or cartilage, fibrous tissue, muscle, adipocytes, ligament, loose connective tissue or granulation tissue depending on environments and situations. That is, woven bones are differentiated into various tissues depending on whether appropriate environments are available or not.
Here, it can be proposed that immediate placement and immediate loading are possible when an implant fixture is placed to maintain lamellar bones as much as possible without being changed to woven bones during dental implantation, or an implant fixture is formed with ideal and most desirable structure to maintain lamellar ones as much as possible. For immediate loading, a bone should be held in close contact with an implant fixture and the state of the bone should also be a lamellar bone. Moreover, the lamellar bone in the horizontal direction should be secured as much as possible. As described above, conventional implant fixtures based on conventional drilling techniques may be structurally damaged by drilling for implantation. This damage changes lamellar bones to woven bones. That is, a dental implant fixture works properly when destroyed lamellar bones are changed to woven bones and are restored to lamellar bones surrounding the dental implant fixture.
A conventional cylindrical implant fixture is disadvantageous for immediate loading due to insufficient structural strength of horizontal bone resisting a vertical force. This is because a force is substantially concentrated on a tip of the cylindrical structure and a screw having a small horizontal area. Disc type dental implant fixtures are advantageous for immediate loading, but the techniques that can precisely place the dental implant fixtures are not good enough for practical application. Now, an explanation will be given concerning the implantation of a typical disc type dental implant fixture claiming to be capable of immediate loading.
In connection with immediate loading, a disc type dental implant fixture of FIG. 1b can be mentioned. Some examples of such disc type dental implant fixtures are found in EP 1 457 165 B1, DE 203 04 367 U1, and EP 0 214 962 B1. FIG. 1b illustrates the shapes of a known dental implant fixture (see FIGS. 8 and 9). The photographs of FIG. 1b show an implantation process of the dental implant fixture, which is the same or similar to a conventional implantation process for immediate loading.
Referring to the photographs of FIG. 1b, the implantation process of the disc type dental implant fixture illustrated in FIGS. 8 and 9 of FIG. 1b is explained below. The lateral sides of an alveolar bone are directly removed by cutting, and the dental implant fixture is immediately implanted and loaded therein. The cutting of the alveolar bone is shown in Photograph 1 and the insertion of the disc type dental implant fixture into the alveolar bone is shown in the other photographs. An accurate position for the implant fixture is secured and an artificial tooth is coupled to the implant fixture for immediate loading, as shown in Photograph 8.
The conventional dental implant fixture is advantageous for immediate loading due to its large horizontal sectional area. However, since implantation of the conventional dental implant fixture proceeds after the lateral sides of alveolar bone are cut, the implantation procedure is complicated and entails much bone loss. Since the alveolar bone cannot be cut precisely in the same form as the conventional dental implant fixture, numerous dead spaces are created. Further, since the dental implant fixture is larger in size than the teeth, the dental implant fixture cannot replace the individual tooth, resulting in a low success rate of implantation. Revision surgery due to this failure leads to extreme bone destruction.
As explained above, the conventional cylindrical dental implant fixtures destroy lamellar bones upon implantation due to their physical structures. Destroyed lamellar bones are changed to woven bones, which are then turned to lamellar bones. In this healing course, the bones become soft and mushy and the dental implant fixtures are not fixed to the bones, making it difficult to couple artificial teeth to the implant fixture. Thus, much time is required for bone regeneration to fix the dental implant fixtures to bones. For this reason, conventional techniques for the implantation of dental implant fixtures are limited to delayed loading. The conventional disc type dental implant fixture has the same problems as described above. Since the conventional disc type dental implant fixture must be buried in bone for delayed loading without any load for a long time, it is coupled to a prosthetic appliance, which is to be connected to a tooth. A problem associated with this structure is the formation of gaps at the connection. The gaps provide an environment for microbial proliferation. For example, microorganisms propagate in the gaps to produce an unpleasant smell and bacterial infection shortens the service life of the dental implant fixture. A rotational force (or a torque) is applied to the dental implant fixture when prosthesis is connected by a connection screw or is disconnected. This force may cause the failure of the dental implant fixture placed with low stability.